Conventionally, a seat using polyester short fibers for a cushion material has been known (for example, see Patent Literature 1 and Patent Literature 2).    Patent Literature 1: Japanese Patent Laid Open No. 08-318066    Patent Literature 2: Japanese Patent Laid Open No. 05-321114
A cushion body used in a seat described in Japanese Patent Laid Open No. 08-318066 is a web obtained by dispersing and incorporating thermally adhesive composite short fibers as adhesive component into matrix fibers composed of an inelastic polyester crimped short fiber assembly, and the web satisfying a relationship of A>3B/2 when the total number of fibers directed in its lengthwise direction is represented as A and the number of fibers directed in its lateral direction is represented as B is in a state of being folded successively in a standing state along its lengthwise direction.
The thermally adhesive composite short fiber is composed of thermoplastic elastomer having a melting point lower than a melting point of polyester polymer constituting a short fiber by 40° C. or higher and inelastic polyester, where the former is at least exposed on a fiber surface.
The cushion body is constituted such that flexible heat-adhesion spots formed by thermally adhering the thermally adhesive composite short fibers in a state of crossing one another and flexible heat-adhesion spots formed by thermally adhering the thermally adhesive composite short fibers and the inelastic polyester short fibers in a state of crossing one another are dispersed in the web when the web is folded in a standing state.
Further, this cushion body is disposed in the seat such that a standing direction of cushioning filler is directed in a direction in which a load is applied.
Since this seat is constituted in such a state that a lengthwise direction of the fibers in the web extends along a load direction, excellent ventilation is, of course, secured, proper hardness to a stress direction is provided, and it becomes easy to disperse the stress.
A cushion body used in a seat described in Japanese Patent Laid Open No. 05-321114 is formed to have a mold face shape molded by disposing a fibrous body composed of a thermoplastic binder which is melted by steam heat and a fibrous group having moderate bulkiness mixed with fibers having a melting point higher than a melting point of this binder between a mold face of a lower mold and a mold face of an upper mold at least one of which has a lot of blowout holes for steam, clamping the both molds, and blowing steam from the blowout holes.
Since the fibrous body including the thermoplastic binder which is melted by the steam heat is disposed in a mold and subjected to steam molding in this manner, it becomes possible to obtain a cushion body having a desired shape.
The art of Patent Literature 1 is preferable to a seat having a two-dimensional structure, but it is insufficient for a seat having a three-dimensional structure. That is, in the art of Patent Literature 1, it is difficult to form a load abutting face into a complex undulation shape or to change cushioning feeling partially, so that a seat producing a good sitting feel has not been able to be obtained.
Further, in the art of Patent Literature 2, the cushion body can be formed into a three-dimensional undulation shape, but it is insufficient regarding hardness in a stress direction, dispersibility of the stress or durability.
There is a problem that a permanent set in fatigue may be generated in a fibrous cushion body due to reception of compression strain in the course of repetitive actual use.
In view of these circumstances, an object of the present invention is to provide a seat having a three-dimensional complex undulated cushion body securing hardness in a stress direction and excelling in dispersibility of the stress and durability and a method of manufacturing the same.
Further, another object of the present invention is to provide a method of treatment for a seat which can recover from a permanent set in fatigue even if the permanent set in fatigue is generated due to that a cushion body of the seat is applied with a load repeatedly.